Understanding Image Histograms and Curves


Image Histograms

Color Channels

Histograms and Pixel Structure

Local Histograms

Using Histograms as a Scanner Tool

Prescan Histograms

Postscan Histograms

Evaluating Histogram Area

Image Exposure and Tone Curves

The Scanning Process

Changing Brightness and Contrast

Color Corrections



Interactive Demos

Setting Exposure

Setting Image Curves


The Photoshop Levels Function and Curves

Why Is There No Luminosity Tool?

Average Skin Tone

Using Histograms to Track Scanner Performance

Further Information

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The purpose of this tutorial was to suggest that the process of using a scanner is greatly simplified if the user approaches it as an exercise in maximizing the quality of data capture. It defined the objectives of this method, the criteria for measuring these objectives, the methods for measurement, and the limiting factors imposed by the hardware and software.  I believe this far more systematic method will result in less frustration and waste of time than the largely intuitive and haphazard approach that most scanner users employ.

Specifically, some of the key elements are that

bulletMost non-professional scanners have limited tonal range sensitivity and we are constrained to making the best of this
bulletHistograms display the tonal characteristics of an image and provide an objective measure of scan quality.  Some of the attributes to be evaluated are the area, contour, and continuity
bulletThe task of the user is to identify the range of meaningful tones of the image and match them to the scanner's capabilities.  This maximizes data capture
bulletIf image-wide changes are necessary, apply tone curves in the scanning step so they will not have to be applied in subsequent uses.

Hopefully, the information in this tutorial will also enable the consumer to evaluate scanners and scanner software more knowledgeably and thus provide vendors further motivation to improve their products.  The user should ask herself or himself whether a prospective product has the features that enable the user to employ the principles outlined here.

Not only are histograms and tone curves powerful tools, but they are absolutely necessary in achieving quality scans.  By comprehending and putting into practice what we have covered, I hope to make scanning a more productive, if not enjoyable, experience.

Scanning Examples